NoClassDefFoundError Minecraft mod stale jar

What this actually means

The JVM found a reference to a class that isn't there at runtime. When the missing class lives in the SAME package as the mod that crashed, it is almost never a dependency problem -- it is a stale or partial jar: the running jar was built before that class existed, or an optional integration class was compiled against a mod that isn't installed, so verification fails the moment it is touched.

How to fix it

• 1. Check you are actually running the jar you think you are. A running
• server keeps the jar it launched with -- a file swap needs a RESTART.
• Compare the version in the jar name against the version in the log.
• 2. If you deploy by copying, DELETE the old jar first. Two versions in
• mods/ or a half-copied file both produce exactly this.
• 3. If the missing class is an 'integration'/'compat' class, it references
• a mod that is not installed. Guard it: only load it after checking
• ModList.get().isLoaded("other_mod"), and never reference its types
• from a class that always loads.
• 4. Rebuild clean (gradlew clean build) -- an incremental build can leave
• a jar missing a class you deleted and re-added.

How to be sure it is this

The missing class is in the SAME package as the code asking for it. That means the jar is incomplete, not that a dependency is absent.

What it gets mistaken for

Looks like a missing dependency. Compare the package names — a dependency failure names someone else's class.

What to do next

Redownload the jar and compare its size against the source. Interrupted downloads are the usual cause, and hosting-panel uploads are a close second.

What it looks like in your log

Search your crash report or latest.log for any of these:

NoClassDefFoundError:
ClassNotFoundException:

The free edition covers the six most common startup failures, and this is not one of them — it is in the paid tool, which checks all 24 and names the mod jar from the stack trace. The fix above is the same one it prints, so you do not need it to solve this.